Bird flu infections in dairy workers went undetected, new study shows

A dairy cow stands in a pen in a large animal containment building at the U.S. Department of Agriculture's National Animal Disease Center research facility in Ames, Iowa, on Tuesday, Aug. 6, 2024. (AP Photo/Charlie Neibergall)

Seven per cent of tested workers on U.S. dairy farms where cows were infected with bird flu caught the virus themselves, according to a new study.

The study proved that more workers were catching bird flu after contact with infected animals than the numbers reflected in official counts, something veterinarians working these farms had warned about since the outbreak began in March.

The research was led by disease detectives at the U.S. Centers for Disease Control and Prevention in partnership with state health departments in Colorado and Michigan. It was published Thursday in the CDC’s Morbidity and Mortality Weekly Report.

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In response to the findings, the agency is expanding its recommendations for testing and treatment of workers on affected farms.

Previously, the CDC recommended testing of only those workers who had symptoms after exposure to infected cows. Now, officials say all workers on farms with infected cows should be tested, even if they don’t have symptoms.

The CDC is also recommending that workers without symptoms receive the antiviral medication Tamiflu if they’ve had a high-risk exposure to infected animals when they weren’t wearing adequate personal protective equipment. An example of a high-risk exposure would be a splash of raw cow’s milk to the face, CDC Principal Deputy Director Dr. Nirav Shah said.

Previously, only workers with symptoms were offered Tamiflu.

Every infection gives the virus an opportunity to change and become more dangerous, so the expanded recommendations are designed to protect workers from the possibility of severe illness if they catch the virus and to limit its spread.

“Simply put, the less room we give this virus to run, the fewer chances it has to cause harm or to change,” Shah said. “And the best way to limit the virus’s room to run is to test, identify, treat and isolate as many cases as possible in humans and as quickly as possible.”

The farm workers who participated in the study were asked about their exposures, any symptoms and their use of personal protective equipment, known as PPE. Their blood was also tested for signs of antibodies, immune proteins that are left behind after the body fights off an infection.

Of 115 people who were tested, eight were positive for antibodies, suggesting that they’d been infected on the job. Only four remembered having any symptoms, however, suggesting that their illnesses may have been very mild. They reported eye redness or conjunctivitis, fever, runny nose, sore throat, sneezing and diarrhea.

All of the workers reported cleaning the milking parlor, the room where cows are milked. Most people who had evidence of infection also said they had milked the cows. None wore the recommended PPE, and only one said they knew that the cows they were working with were infected with H5N1.

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All eight workers who tested positive for past infections spoke Spanish. The CDC’s recommendations stress increasing education for workers on farms, including culturally appropriate guidance in their native languages.

There have been 46 human cases of H5N1 reported in the United States this year. The eight workers identified in the new study will not be added to the case count, officials said. A case must be detected while an infection is active to be included in the total.

“They are really looking retrospectively, helping us learn, but they’re not going to be called cases,” said Dr. Demetre Daskalakis, director of the CDC’s National Center for Immunization and Respiratory Diseases.

The CDC said the findings of the study — including limited use of PPE by dairy workers and lack of awareness of the infection in cattle — point to the need for more outreach to this vulnerable group.

“These were farms with known infected cows,” Daskalakis said. “So it was really about the fact that they didn’t have the awareness that they had any sick cows that they were working with, even though they were in environments where sick cows were known.”

The CDC has said that it is working closely with states and local health officials as it continues to monitor the H5 bird flu situation and that the current risk to the public remains low.

“The focus is going to be on containing the spread of infection in cows, because that is believed to be the best way to address controlling spread to farm workers,” Lori Tremmel Freeman, chief executive officer for the National Association of County and City Health Officials, said Thursday.

“This disease is interesting because the more we learn, the more questions come up. But I think there are good tactics being used to address this.”

CNN’s Jacqueline Howard contributed to this report.